1. Field of Invention
The present invention relates to hydrostatic transmissions intended primarily for use in the lawn and garden industry on vehicles such as riding lawnmowers and wide area, walk-behind lawnmowers, particularly lawnmowers of the zero turn radius variety which are steered by controlling the speed and direction of their ground-engaging drive wheels.
2. Description of the Related Art
Zero turn radius mowers have long been favored by landscape maintenance professionals for their high level of maneuverability; mowers of this type may be turned in place through 360xc2x0, allowing faster lawn cutting times. Further, relatively less expensive, noncommercial versions of these mowers have recently become available for residential use. These mowers are provided with a pair of reversible drive wheels, one on each side of the mower, which are independently controlled. Each wheel is driven by a separate fluid motor, and each motor is driven by a separate, positive displacement fluid pump. Normally, there are gear reduction means between the motor and the wheel, by which the speed of the wheel is reduced, and its output torque increased. An engine drives the pumps at a constant speed, and the speed and direction of rotation of each wheel motor is individually controlled by altering the displacement of its associated pump and selecting which of two conduits fluid flows from the pump to the motor. Rotation of the drive wheels in opposite directions at common speeds will spin the mower in place relative to the ground, thereby providing a zero turn radius.
A fluid pump and motor pair may be comprised of separate pump and motor units in fluid communication through hydraulic fluid lines. The axle and gear reduction means may comprise another separate unit attached to the motor unit. A pump and motor pair is required for driving each axle. Additionally, a separate fluid reservoir or sump is also necessary to provide any needed hydraulic fluid to the pump, and space on the mower must be provided for packaging the reservoir or sump.
It is necessary that the engine and pump positions, and the motor and drive axle positions, be fixed relative to each other. Previous zero turn radius mowers rely on the mower frame, which may be made of heavy gauge sheet steel, plate steel and/or angle iron, to provide structural rigidity not only between the engine and the pump units, but also between the two wheel motor units. To provide such rigidity, the frame may require substantial bracing. Further, these frames must be provided with closely toleranced mounting holes for mounting the individual pumps, motors and gear reduction means. Thus previous zero turn radius mowers require a frame which is heavy, rigid, and expensive.
It is known by those of ordinary skill in the art to package a fluid pump and motor pair in a common housing, thereby eliminating external hydraulic lines therebetween and providing a compact module which also includes the reservoir or sump. Examples of such a hydrostatic transmission modules are described in U.S. Pat. No. 6,301,885 and in pending U.S. patent application Ser. No. 09/498,692, filed Feb. 7, 2000, and Ser. No. 09/671,796, filed Sep. 27, 2000, all of which are assigned to Tecumseh Products Company, the disclosures of which are each expressly incorporated herein by reference.
Also known in the art are hydrostatic axle drive units in which a fluid pump and motor pair are packaged with gear reduction means and a portion of an axle shaft in a common housing. One example of such an axle drive is Model 310-1400 IZT (Integrated Zero-Turn) transaxle manufactured by Hydro-Gear of Sullivan, Ill. One IZT transaxle unit is attached to each side of the mower, and drives one of the two drive wheels.
Separate axle drive units such as the IZT transaxle are rather large, and occupy a considerable amount of space on the mower. Further, separate axle drive units such as the IZT transaxle still rely on the mower frame to provide structural rigidity between the two axle drive units. Thus a heavy, rigid and expensive frame is still required to accommodate a pair of individual axle drive units such as IZT transaxles. Additionally, two of these large axle drive units per mower must be shipped, stored and inventoried by the mower assembler.
Moreover, in mowers having a pair of axle drive units such as IZT transaxles, hydrostatic component failure (e.g., failure of the fluid pump or motor) requires removal of an entire axle drive unit, and its associated drive wheel, from the mower, rendering the mower completely immobile, and involves a considerable amount of time to accomplish direct replacement of the failed axle drive unit with another such unit.
What is needed is a compact hydrostatic transmission unit for zero turn radius mowers which has built-in structural rigidity, thereby accommodating a lighter and less expensive frame than would otherwise be necessary, and which, vis-a-vis previous hydrostatic transmissions, facilitates a simpler zero turn radius mower assembly process, reduces shipping and inventory administration costs for such mowers, and provides a means for easily moving a mower having failed hydrostatic components and replacing those components easily and quickly.
The present invention overcomes the above-mentioned shortcomings of previous hydrostatic transmissions for zero turn radius mowers. The inventive transaxle provides the advantages of requiring only a single, compact transaxle unit per mower, which has built-in rigidity between the paired pumps and motors, which may be easily moved in the event of hydrostatic component failure, and which has hydrostatic components which may be easily and quickly replaced without rendering the mower even temporarily completely immobile.
The present invention provides a hydrostatic transaxle including first and second hydrostatic transmission modules and an axle drive housing. Each module includes a transmission housing, a rotating fluid pump and a rotatable fluid motor with the pump being of variable displacement and the motor being rotated at various speeds and in forward and reverse directions in response to changes in pump displacement. The axle drive housing contains first and second gear trains and a pair of axles. One of the axles is operably coupled to the first transmission module motor through the first gear train, and the other of the axles is operably coupled to the second transmission module motor through the second gear train. Each of the hydrostatic transmission module housings is detachably fixed to the axle drive housing.
By providing built-in structural rigidity between the pumps and motors, the inventive transaxle may be accommodated by a frame which is comparatively lighter, cheaper and less rigid than previously required for a zero turn radius mower. Further, the inventive transaxle provides the advantage of selectively coupling the motors to their associated gear trains, thereby allowing the mower to be easily moved in the event of a hydrostatic component failure. Further still, the inventive transaxle provides the advantage of allowing removal and replacement of the hydrostatic transmission modules without removal of the axle drive housing, either axle or either drive wheel; consequently, the mower is not rendered completely immobile due to a hydrostatic component failure. Moreover, the present invention provides a single compact hydrostatic transaxle through which a zero turn radius mower can be driven; the inventive transaxle is approximately the size of one of the two IZT transaxle units required per mower.
The present invention further provides a zero turn radius vehicle including a frame, a transaxle attached to the frame, and a pair of ground engaging wheels. The transaxle includes first and second hydrostatic transmission modules and an axle drive housing. Each module includes a transmission housing, a rotating fluid pump and a rotatable fluid motor with the pump being of variable displacement and the motor being rotated at various speeds and in forward and reverse directions in response to changes in pump displacement. The axle drive housing contains first and second gear trains and a pair of axles. One of the axles is operably coupled to the first transmission module motor through the first gear train, and the other of the axles is operably coupled to the second transmission module motor through the second gear train. Each of the hydrostatic transmission module housings is detachably fixed to the axle drive housing. One of the ground-engaging wheels is connected to each the axle.
The present invention further provides a hydrostatic transaxle for use in a zero turn radius mower having a rotating blade including a first and a second hydrostatic transmission module and an axle drive. The first and second hydrostatic transmission modules are substantially mirror image versions of each other, and include a transmission housing, a variable displacement rotating fluid pump disposed within the transmission housing, and a rotatable fluid motor disposed within the transmission housing and in fluid communication with the pump, the motor being rotated at various speeds and in forward and reverse directions in response to changes in pump displacement. The first and the second transmission modules are detachably fixed to the axle drive. The axle drive includes an axle drive housing, a first reduction means and a second reduction means disposed in the axle drive housing, and a pair of axles rotatably supported in the axle drive housing. One of the axles is operably coupled to the first transmission module motor through the first reduction means, and the other of the axles is operably coupled to the second transmission module motor through the second reduction means.